This invention relates to fluid translating units such as fluid pumps and motors of the type having reciprocating hollow-skirt pistons. In particular this invention relates to improved retaining means for retaining a core of filler material in the hollow skirt of such pistons, and a method for making such retaining means.
Conventionally, the piston employed in reciprocating-piston fluid translating units contain cavities in the body or skirt portions thereof formed by removing a core of the piston material to reduce the weight of the piston and thus reduce the high inertia forces acting on the piston during operation of the units. This, however, increases the dead volume of fluid which must be moved within the translating unit and tends to decrease the efficiency of these units. Consequently, it is usual to refill these cavities with a material which is lighter in weight than the piston material, but which also possesses a sufficiently high bulk modulus to resist compression under the high pressures produced within these translating units. Such filler material must be positively retained within the piston to prevent displacement during operation of the units and thereby avoid potential severe damage thereto.
Particularly suitable prior art means for retaining a core of filler material within a piston cavity comprise those means for mechanically interlocking the core and piston described in U.S. Pat. Nos. 3,707,113; 1,204,889; 1,433,478; 2,741,518; 3,080,854 and 3,187,644; as well as German Patent No. 922,038 (1955). Other means include those means described in patent application Ser. Nos. 391,337 and 393,045 and 371,666 of common assignment herewith. Retaining means of the mechanically interlocking type have, however, in some instances proven to be relatively expensive to fabricate, and it is therefore desirable to provide retaining means for such filler material which are effective and yet do not incur high attendant fabrication costs.